Light Emitting Diodes (LED) light fixtures are becoming commonplace as utilities, governments, businesses, and individuals seek methods of decreasing energy costs. LED lights have the advantage of decreased energy usage when compared to traditional light sources such as incandescent, metal halide, and high pressure sodium. Additionally, with projected lives of 100,000 hours or more, they provide the ideal replacement where maintenance costs are high, such as street lighting.
Typically, an LED fixture comprises a housing, an LED light source, a lens, a bezel for securing the lens to the housing, and a gasket for creating a seal between the housing and the bezel. Creating a sealed fixture is particularly important when the fixture will be exposed to harsh environments, such as weather when the fixture is used for outdoor or street lighting. Traditionally, the lens, gasket, and bezel are separate components and must be preassembled before securing to the fixture. A one piece lens, gasket, and bezel would reduce assembly cost by eliminating the preassembly step.
Plastic or polymer components, including the lens and gasket, and sometimes the bezel, are typically injection molded. In the injection molding process, material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity. Molds can be of a single cavity or multiple cavities. In multiple cavity molds, each cavity can be identical and form the same parts or can be unique and form multiple different parts during a single cycle. Molds are generally made from tool steel, but stainless steels and aluminum molds are suitable for certain applications.
A parting line, sprue, gate marks, and ejector pin marks are usually present on the final part. None of these features are typically desired, but are unavoidable due to the nature of the process. Gate marks occurs at the gate which joins the melt-delivery channels (sprue and runner) to the part forming cavity. Parting line and ejector pin marks result from minute misalignments, wear, gaseous vents, clearances for adjacent parts in relative motion, and/or dimensional differences of the mating surfaces contacting the injected polymer. Dimensional differences can be attributed to non-uniform, pressure-induced deformation during injection, machining tolerances, and non-uniform thermal expansion and contraction of mold components, which experience rapid cycling during the injection, packing, cooling, and ejection phases of the process. Mold components are often designed with materials of various coefficients of thermal expansion.
The mold consists of two primary components, the injection mold and the ejection mold. Plastic resin enters the mold through a sprue in the injection mold and a sprue bushing on the mold tightly seals against the nozzle of the injection barrel of the molding machine to allow molten plastic to flow from the barrel into the mold. The sprue bushing directs the molten plastic to the cavity images through channels that are machined into the faces of the injection and ejection molds. These channels allow plastic to run along them, so they are referred to as runners. The molten plastic flows through the runner, enters one or more gates, and flows into the cavity to form the desired part.
The amount of resin required to fill the sprue, runner and cavities of a mold is a shot. Trapped air in the mold can escape through air vents that are ground into the parting line of the mold. If the trapped air is not allowed to escape, it is compressed by the pressure of the incoming material and is squeezed into the corners of the cavity, where it prevents filling and causes other defects as well.
Sides of the part that appear parallel with the direction of draw are typically angled slightly (referred to as draft) to ease release of the part from the mold. The draft required for mold release is primarily dependent on the depth of the cavity: the deeper the cavity, the more draft necessary. Shrinkage must also be taken into account when determining the draft required. If the skin is too thin, then the molded part will tend to shrink onto the cores that form them while cooling and cling to those cores. Then the part may warp, twist, blister or crack when the cavity is pulled away. The mold is usually designed so that the molded part reliably remains on the ejection mold of the mold when it opens, and draws the runner and the sprue out of the injection mold along with the part. The part then falls freely when ejected from the ejection mold. Ejector pins, also known as knockout pins, are circular pins placed in either half of the mold (usually the ejection half), which push the finished molded product of a mold after it has cooled sufficiently.
The standard method of cooling is passing a coolant (usually water) through a series of holes drilled through the mold plates and connected by hoses to form a continuous pathway. The coolant absorbs heat from the mold (which has absorbed heat from the hot plastic) and keeps the mold at a proper temperature to solidify the plastic at the most efficient rate.
Over-molding refers to inserting previously molded parts into an injection molding machine to inject a new plastic or polymeric layer around the first part. Co-molding refers to molding multiple components of the same assembly in the injection molding machine, typically at the same time, either with the same mold or with multiple molds.
Two-shot or multi-shot molds are designed to co-mold within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This process is actually an injection molding process performed twice. In the first step, the first material molded into a basic shape, which contains spaces for the second shot. Then the second material is injection-molded into those spaces and adheres to the first material.